1. Field of the Invention
This invention relates to an electroluminescent device, and more particularly, to an electroluminescent device having a luminescent layer comprising two layers each of which comprises a thin film of an electroluminescent organic compound, in particular, at least one layer being a thin film of a highly ordered molecular orientation, each of the two layers is electron-acceptable or electron donative relative to the other layer.
2. Description of the Prior Art
Conventional electroluminescent devices (hereinafter sometimes referred to as "EL devices") are composed of a luminescent layer comprising a luminescent matrix of ZnS containing Mn, Cu, or ReF.sub.3 (Re: rare earth ion) as an activating agent. Electroluminescent devices are generally classified into two structural types, i.e. powder type EL device and thin film type EL device based on the type of the fundamental structure of the luminescent layer.
Among practical electroluminescent devices, the thin film type EL generally exhibits a high luminance as compared with the powder type EL, but can be produced in the form of a large area element with great difficulty since the thin film type EL is fabricated such that a luminescent matrix is deposited on a substrate to form the luminescent layer, and moreover, the production cost is disadvantageously very high. Therefore, powder type EL has recently drawn attention since it is suitable for mass production and can be produced by dispersing a luminescent matrix such as ZnS in an organic binder resulting in a low production cost which is a few tens fraction of the production cost of thin film type devices.
In general, the thinner the luminescent layer, the better the luminescence characteristics, in the case of electroluminescence. However, luminescent matrix for powder type EL is discontinuous powders and when the thickness of the luminescent layer is thin, pinholes are liable to be formed in the luminescent layer and thereby it is difficult to make the layer thickness thin and satisfactory luminance characteristics can not be obtained. Japanese Patent Application Laid-open No. 172891/1983 discloses an improved electroluminescent element comprising an intermediate dielectric layer composed of a polymer of vinylidene fluoride in a luminescent layer of powder type EL. However, satisfactory luminance and desirably low power consumption have not yet been achieved.
On the other hand, there have been recently made active researches and developments that new optical and electronics materials are produced by controlling chemical structure and high order structure of organic materials. As the result, there have been proposed EC device, piezoelectric device, pyroelectric element, nonlinear optical device, ferroelectric liquid crystal and the like made of organic materials which are comparable to or better than those made of metals or inorganic materials. In such a manner as above, development of functional organic materials as new functional materials surpassing inorganic materials are now demanded.
Example of devices made of organic materials are disclosed in Japanese Patent Application Laid-open No. 35587/1977, that is, EL devices produced by forming a built-up film of monomolecular layers of anthracene derivatives or pyrene derivatives having both hydrophilic and hydrophobic substituent groupings in the molecule on electrode substrates. However, such EL devices have not yet achieved a sufficient luminance and low power consumption which are necessary conditions for practical EL devices. Further, in such organic EL devices, density of carrier electron or hole is so small that excitation probability of functional molecules caused by recombination of the carriers is very low and therefore an efficient luminescence can not be expected.